1. Field of Invention
The invention relates to replication of data in a computer system. More particularly, the present invention relates to utilizing the SCSI transport layer over TCP/IP, via an IP multicast mechanism, in order to replicate data transferred to a SCSI device.
2. Description of the Related Art
The Small Computer Systems Interface (“SCSI”) is a popular family of protocols for communicating with I/O devices, in particular storage devices. More to the point, SCSI is the basic protocol for I/O operations between computers and storage subsystems. It is, in essence, a point-to-point protocol.
Another popular protocol is the Transport Control Protocol/Interface Program (“TCP/IP”). TCP/IP is the basic set of communication protocols for general data exchange between computers connected on a communication network. This is a common protocol used to communicate via the Internet.
Currently there is a convergence between the two protocols, that is, SCSI and TCP/IP. In particular, computers that communicate with their subsystems via SCSI are now tending to be interconnected via the Internet and utilizing TCP/IP to communicate with each other.
In view of this convergence, a standard has been proposed for implementing SCSI over TCP. The currently proposed standard “-draft-ietf-ips-iSCSI-06.txt” is available at http://www.ietf.org/internet-drafts/draft-ietf-ips-iscsi-06.txt (herein expressly incorporated by reference) (the “Standard”). The Standard aims to be fully compliant with the requirements laid out in the SCSI Architecture Model-2 Document (also herein expressly incorporated by reference). This Standard provides for transmitting SCSI commands between SCSI devices, over TCP/IP connections. Conventionally, SCSI devices that communicate with each other must be connected via the same SCSI bus. The proposed Standard permits SCSI devices which are not connected via the same SCSI bus to communicate with each other via the Internet, utilizing TCP/IP protocol. According to the Standard, SCSI devices that are separated from one another even by great distances may communicate with each other. The Standard seeks to describe an implementation of SCSI that operates on top of TCP as the transport protocol.
There are numerous higher level storage functions that, in the context of a SCSI protocol, are implemented as a set of point-to-point I/O operations. One of the most well known of these higher level storage functions is Data Mirroring. In Data Mirroring on a conventional SCSI system, data replication is implemented by issuing separate SCSI I/O operations to each device that is intended hold an image of the replicated data.
The following briefly defines some SCSI conceptual terms. The end point of a typical SCSI command is a “logical unit” (LUN). Logical units include, for example, hard drives, tape drives, CD and DVD drives, printers and processors. A collection of logical units is referred to as a “target” and is directly addressable on the network. In a client-server model, the target corresponds to the server. An “initiator” creates and sends SCSI commands to the target. In the client-server model, the initiator corresponds to the client.
A typical SCSI command results in a command phase, data phase and a response phase. In the data phase, information travels either from the initiator to the target (for example, a WRITE command), or from the target to the initiator (for example, a READ command). In the response phase, the target returns the final status of the operation, including any errors. A response signals the end of a typical SCSI command.
In a conventional storage subsystem, data mirroring is accomplished by providing additional logic, generally on the I/O operation level. Typically, for each WRITE operation for data which is mirrored, this additional logic issues a corresponding WRITE operation to every device that maintains the mirrored image of the data. Thus, if a conventional system provides for two-way mirroring, two copies of each WRITE command are sent over the SCSI bus; moreover, two copies of the data associated with the WRITE command are sent over the SCSI bus, resulting in multiple transfers of the same data. Consider, for example, that WRITE commands from an initiator such as a personal computer are to be mirrored to three targets such as three data storage devices, e.g., ZIP drives. Every time the initiator generates a WRITE command which is to be mirrored, the WRITE command and associated data must be sent to all three of the targets. Moreover, the I/O subsystem requires additional complicated logic in order to handle the data replication, error handling and data routing within the SCSI network. As can be seen from foregoing, data mirroring typically dramatically increases the use of the SCSI bus. This, in turn, results in a significantly slower throughput to other devices utilizing the SCSI bus and increases the probability of errors.
FIG. 1 is an illustration of a typical remote data mirroring system, shown in U.S. Pat. No. 6,173,377 B1, to Yanai et al. This system automatically provides and maintains a copy or mirror of data stored at a location geographically remote from the main or primary data storage device. The data processing system 110 in FIG. 1 can be configured for remote mirroring from a user interface of the service processor 134 in the primary data storage system 114. Remote mirroring software 113 can be provided; host application programs can interface with a remote mirroring facility of the data storage systems 114, 146 via the remote mirroring software 113. According to Yanai process and system, central processing unit 152 could be located with the remote secondary data storage system 146, linked to the primary and secondary data storage systems 114, 146 via redundant signal paths. Communication links 140, 141 from link adapters 136, 137 are provided. To provide remote mirroring operating modes for specific applications, the remote mirroring facility defines an operating mode for each logical volume of data in the storage device as in the data storage systems 114, 146. A specific set of methods are provided in order to remotely mirror data according to either a synchronous or a semi-synchronous mode. When a host writes data to a remotely mirrored volume, a series of steps occurs. Data is written to the cache of the data storage system with the primary volume; an entry is queued up to transmit data to the secondary volume; the secondary volume's data storage system acknowledges receipt of the data; and a write end signal is returned to the host that initiated the WRITE request.